At present, satellite location of a mobile receiver is effected by the measurement of pseudo-distances. These measurements are generally biased by the clock error of the receiver. This error can be eliminated provided that an additional measurement is added, i.e. provided that at least four satellite measurements are available.
The pseudo-distance measurements are subject to errors linked to the receiver, caused in particular by thermal noise and by propagation in the atmosphere that causes different time-delays according to the satellites.
One of the dominant sources of error is the time-delay caused by the ionosphere. Nevertheless, there exist models, such as the models known as “Klobuchar” or “Nequick”, providing corrections as a function of the position of the receiver and the satellites, but these can at best correct 50% of errors.
In prior art solutions, location of a global navigation satellite system (GNSS) receiver employs satellite signals that are generally single-frequency signals. The measurements of the position of a GNSS receiver are then limited in terms of accuracy.
A powerful technique for correcting the ionospheric error consists in sending two signals per satellite on different carrier frequencies. The ionospheric delay being inversely proportional to the square of the frequency, the propagation time difference observed between the two frequencies can be used to work back to the ionospheric error and to subtract it from the measurements.
The current GPS system uses signals L1 and L2 that can benefit from this error correction technique. On the other hand, access to the signals L2 is effected either via encrypted codes or via a “semi-codeless” technique offering lower performance. Moreover, the L2 band is not certified for aeronautical applications, and so this method is not used at present for civil aviation, for example.
The deployment of GNSS systems, such as the Galileo and modernized GPS systems, will generalize two-frequency signals in the ARNSS (Aeronautical Radio Navigation Satellite Service) bands. It will further enable all receivers to offer the possibility of correcting ionospheric errors to improve GNSS receiver positioning.
Existing two-frequency receivers use two separate analogue channels a and b to process the two frequencies Fa and Fb. For example, GPS receivers for civil aviation use the frequency L1 substantially equal to 1575.42 MHz and the frequency L5 substantially equal to 1176.45 MHz. Galileo receivers use the frequency L1 and the frequency E5b substantially equal to 1207.14 MHz.
Whatever frequencies are used, when the two-frequency receivers receive two signals, the propagation times Ba and Bb in each of the analogue channels of the receiver can be different. The effect of this is to introduce a bias into the estimate of the ionospheric error for each satellite based on the propagation time difference between the two frequencies.
If all the measurements used to resolve the position of two-frequency signals, and this bias is thus found to be identical for all the satellites, there is no effect on the position. In this case the resolution position, velocity and time (PVT) algorithm introduces this bias on account of the clock error of the receiver.
However, certain situations, concerning the determination of the position of a GNSS receiver, necessitate the ability to use mixed single-frequency and two-frequency measurements to resolve the position of the receiver.
These situations arise in particular during the transient phase of replacement of the satellites of a constellation, the old single-frequency satellites being progressively replaced by two-frequency satellites.
These situations also arise in the case of interference in one of the two bands, notably if the level of interference has the consequence of causing a few of the weakest satellites to become desynchronized. Ionospheric scintillation can momentarily desynchronize one of the two frequencies on some satellites.
The choice to use single-frequency measurements, less accurate because of the ionospheric correction based on a model, at the same time as two-frequency measurements is justified by the resulting improvement in terms of satellite geometry, also known as GDOP (geometric dilution of precision).
The single-frequency measurements must nevertheless be weighted with a variance representative of the real error including the residual ionospheric error after application of the ionospheric model.
However, although single-frequency and two-frequency satellite measurements enable the position of a GNSS receiver to be determined, the inter-frequency bias in the HF channels of a two-frequency receiver can induce a high error in the resolved position.
One object of the invention is to alleviate the aforementioned drawbacks.